The present invention relates to the manufacture of substrates. More particularly, the invention provides a technique including a method and device for cleaving a substrate in the fabrication of a silicon-on-insulator substrate for semiconductor integrated circuits using a pressurized fluid, for example. But it will be recognized that the invention has a wider range of applicability; it can also be applied to other substrates for multi-layered integrated circuit devices, three-dimensional packaging of integrated semiconductor devices, photonic devices, piezoelectronic devices, microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d), sensors, actuators, solar cells, flat panel displays (e.g., LCD, AMLCD), biological and biomedical devices, and the like.
Craftsmen or more properly crafts-people have been building useful articles, tools, or devices using less useful materials for numerous years. In some cases, articles are assembled by way of smaller elements or building blocks. Alternatively, less useful articles are separated into smaller pieces to improve their utility. A common example of these articles to be separated include substrate structures such as a glass plate, a diamond, a semiconductor substrate, and others.
These substrate structures are often cleaved or separated using a variety of techniques. In some cases, the substrates can be cleaved using a saw operation. The saw operation generally relies upon a rotating blade or tool, which cuts through the substrate material to separate the substrate material into two pieces. This technique, however, is often extremely xe2x80x9croughxe2x80x9d and cannot generally be used for providing precision separations in the substrate for the manufacture of fine tools and assemblies. Additionally, the saw operation often has difficulty separating or cutting extremely hard and/or brittle materials such as diamond or glass.
Accordingly, techniques have been developed to separate these hard and/or brittle materials using cleaving approaches. In diamond cutting, for example, an intense directional thermal/mechanical impulse is directed preferentially along a crystallographic plane of a diamond material. This thermal/mechanical impulse generally causes a cleave front to propagate along major crystallographic planes, where cleaving occurs when an energy level from the thermal/mechanical impulse exceeds the fracture energy level along the chosen crystallographic plane.
In glass cutting, a scribe line using a tool is often impressed in a preferred direction on the glass material, which is generally amorphous in character. The scribe line causes a higher stress area surrounding the amorphous glass material. Mechanical force is placed on each side of the scribe line, which increases stress along the scribe line until the glass material fractures, preferably along the scribe line. This fracture completes the cleaving process of the glass, which can be used in a variety of applications including households.
Although the techniques described above are satisfactory, for the most part, as applied to cutting diamonds or household glass, they have severe limitations in the fabrication of small complex structures or precision workpieces. For instance, the above techniques are often xe2x80x9croughxe2x80x9d and cannot be used with great precision in fabrication of small and delicate machine tools, electronic devices, or the like. Additionally, the above techniques may be useful for separating one large plane of glass from another, but are often ineffective for splitting off, shaving, or stripping a thin film of material from a larger substrate. Furthermore, the above techniques may often cause more than one cleave front, which join along slightly different planes, which is highly undesirable for precision cutting applications. Other processing techniques such as the use of a release layer have also had limited success. Such release layer techniques often require wet chemical etching, which is often undesirable in many state of art applications.
From the above, it is seen that a technique for separating a thin film of material from a substrate which is cost effective and efficient is often desirable.
According to the present invention, an improved technique for removing a thin film of material from a substrate using a controlled cleaving action is provided. This technique allows an initiation of a cleaving process on a substrate using a single or multiple cleave region(s) through the use of controlled energy (e.g., spatial distribution) and selected conditions to allow an initiation of a cleave front(s) and to allow it to propagate through the substrate to remove a thin film of material from the substrate.
In a specific embodiment, the present invention provides a process for forming a film of material from substrates. The process includes providing a bulk substrate (e.g., silicon) comprising an upper surface. The process also includes forming a compressive layer (e.g., silicon germanium) overlying the upper surface. The method also includes forming a material layer (e.g., epitaxial silicon) overlying the compressive layer. Energy is provided using a fluid to a selected region of the compressive layer to initiate and maintain a controlled cleaving action at the compressive layer, whereupon the cleaving action is made using a propagating cleave front to free the material layer from the bulk substrate. Preferably, the bulk substrate structure is bonded to a handle wafer, where the device layer overlies the handle wafer, before the energy is provided.
In an alternative embodiment, the present invention provides a process for forming a film of material from substrates. The process includes providing a bulk substrate (e.g., silicon) comprising an upper surface. The process also includes forming a cleaving layer overlying the upper surface. The process also includes forming a material layer (e.g., epitaxial silicon) overlying the cleaving layer. Next, particles (e.g., hydrogen, helium, deuterium, or any other light atomic mass particles and the like) are introduced into the cleaving layer. These particles enhance the present cleaving action, which is described below. Energy is provided using a fluid to a selected region of the cleaving layer to initiate and maintain a controlled cleaving action at the cleaving layer, whereupon the cleaving action is made using a propagating cleave front to free the material layer from the bulk substrate. In a specific embodiment, cleaving occurs at or within the cleaving layer.
In most of the embodiments, a cleave is initiated by subjecting the material with sufficient energy to fracture the material in one region or more, causing a cleave front, without uncontrolled shattering or cracking. The cleave front formation energy (Ec) must often be made lower than the bulk material fracture energy (Emat) at each region to avoid shattering or cracking the material. The directional energy impulse vector in diamond cutting or the scribe line in glass cutting are, for example, the means in which the cleave energy is reduced to allow the controlled creation and propagation of a cleave front. The cleave front is in itself a higher stress region and once created, its propagation requires a lower energy to further cleave the material from this initial region of fracture. The energy required to propagate the cleave front is called the cleave front propagation energy (Ep). The relationship can be expressed as:
Ec=Ep+[cleave front stress energy]
A controlled cleaving process is realized by reducing Ep along a favored direction(s) above all others and limiting the available energy to be below the Ep of other undesired directions. In any cleave process, a better cleave surface finish occurs when the cleave process occurs through only one expanding cleave front, although multiple cleave fronts do work.
Numerous benefits are achieved over pre-existing techniques using the present invention. In particular, the present invention uses controlled energy and selected conditions to preferentially cleave a thin film of material from a donor substrate which includes multi-material sandwiched films. This cleaving process selectively removes the thin film of material from the substrate while preventing a possibility of damage to the film or a remaining portion of the substrate. Accordingly, the remaining substrate portion can be re-used repeatedly for other applications.
Additionally, the present invention uses a relatively low temperature during the controlled cleaving process of the thin film to reduce temperature excursions of the separated film, donor substrate, or multi-material films according to other embodiments. In most cases, the controlled cleaving process can occur at, for example, room temperature, as well as others. This lower temperature approach allows for more material and process latitude such as, for example, cleaving and bonding of materials having substantially different thermal expansion coefficients. In other embodiments, the present invention limits energy or stress in the substrate to a value below a cleave initiation energy, which generally removes a possibility of creating random cleave initiation sites or fronts. This reduces cleave damage (e.g., pits, crystalline defects, breakage, cracks, steps, voids, excessive roughness) often caused in pre-existing techniques. Moreover, the present invention reduces damage caused by higher than necessary stress or pressure effects and nucleation sites caused by the energetic particles as compared to pre-existing techniques. Depending upon the embodiment, one or more of these advantages, as well as others, may exist.
The present invention achieves these benefits and others in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.